1. Field of the Invention
The present invention relates to a double tunnel junction structure used as a tunnel junction sensor in a magnetic head, and more particularly, to a double tunnel junction structure having enhancement layers that boost the magnetoresistance with multiple barriers used to eliminate the effect of the applied dc bias without reduction in spin polarized tunneling.
2. Description of the Related Art
A read head employing a read sensor may be combined with an inductive write head to form a combined magnetic head. In a magnetic disk drive, an air bearing surface (ABS) of the combined magnetic head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode, the resistance of the read sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a current is conducted through the read sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A read sensor is employed by a magnetic head for sensing magnetic fields from moving magnetic media, such as a magnetic disk or a magnetic tape. One type of read sensor employs a tunnel junction sensor. The typical tunnel junction sensor includes a nonmagnetic spacer layer sandwiched between first and second ferromagnetic layers, commonly called a pinned layer, and a free layer. The magnetization of the pinned layer is pinned 90.degree. to the magnetization of the free layer and the magnetization of the free layer is free to respond to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic pinning layer.
The tunnel junction sensor is based on the phenomenon of spin-polarized electron tunneling. The typical tunnel junction sensor uses ferromagnetic metal electrodes, such as NiFe or CoFe, having high coercivity with a spacer layer that is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers (FM/IFM). The tunneling phenomenon is electron spin dependent, making the magnetic response of the tunnel junction sensor a function of the relative orientations and spin polarization of the two ferromagnetic layers. The details of tunnel junction structures have been described in the commonly assigned U.S. Pat. No. 5,650,958 to Gallagher et al., which is incorporated by reference herein.
FIG. 1 shows tunnel magnetoresistance (TMR) as a function of dc bias for a tunnel junction sensor. At low dc bias, the conduction varies only slightly with the dc bias. As the dc bias increases, the TMR coefficient drops noticeably. For example, the application of 300 mV bias across a tunnel junction structure having a structure comprising ferromagnetic/insulator/ferromagnetic (FM/I/FM) reduces the TMR by half.
To solve this problem, another type of tunnel junction sensor has been proposed called a double junction sensor (FM/I/FM/I/FM). FIG. 2 shows a prior art tunnel junction sensor 200 which includes a first pinning layer 205, a first pinned layer 210, a first spacer layer 215, a free layer 220, a second spacer layer 225, a second pinned layer 230 and a second pinning layer 235. The magnetization of the outer two FM pinned layers are parallel while the magnetization of the internal FM free layer is either parallel or antiparallel. Modeling has shown that the double tunnel junction behaves differently than the traditional single tunnel junction by eliminating the effect of dc bias. FIG. 3 shows the TMR as a function of the dc bias for a double junction tunnel junction sensor.
While it appears that the multiple barriers have been shown to significantly eliminate the effect of dc bias, the double tunnel junction has drawbacks. For the spin polarized resonant tunneling phenomenon to work, the layers of the double tunnel junction must be made very thin. While it is desired to have thin layers, too thin a layer is detrimental to the device. For example, the center FM layer (traditionally the free layer) for the prior art is between 10 and 20 .ANG.. With a layer this thin, the ferromagnetic free layer becomes saturated easily from external magnetic fields. Once saturated, the double tunnel junction sensor does not get the full benefit of the ferromagnetic free layer, the signals get clipped. It is preferable that the free layer never be saturated.
From the above discussion it becomes apparent that what is needed is a double tunnel junction sensor that provides the benefits of improved spin polarized tunneling and minimizing dc bias effects while also providing a device in which the internal layers are not saturated by an external magnetic field.